"The Tokaimura nuclear fuel processing plant is operated by JCO Company
Ltd. and is located approximately 120 kilometers northeast
of Tokyo.... The main function of the JCO plant is
to convert isotopically enriched uranium hexafluoride into uranium
dioxide fuel.... The uranium
used in the process has been enriched to contain up to 5% of the
fissile isotope, 235U.

[A standard process at the plant was to treat] high-purity enriched
uranium oxide
with nitric acid to form uranyl nitrate for shipping. On the morning of
September 30,
1999, three technicians, Hisashi Ouchi, Masato Shinohara, and Yutaka
Yokokawa, were running fuel through the last steps of the conversion
process. To speed up the process, they mixed the oxide and nitric acid
in 10-liter stainless steel buckets rather than in the dissolving tank.
For
convenience, they added the bucket contents directly to the ...
precipitation tank rather than to the
buffer tank. That was a crucial error because the tall, narrow geometry
of the buffer tank was designed to preclude the onset of criticality.
In filling the precipitation tank, the crew added ... about 16 kg of
enriched uranium....

Masato
Shinohara stood on a platform and was pouring the uranyl nitrate
solution into the precipitation tank while Hisashi Ouchi held a glass
funnel in an inlet at the top of the tank. [Yutaka Yokokawa was seated
at a desk]. At approximately 10:35 a.m. the
technicians added the seventh bucket and saw a blue flash. The two
technicians near the vessel began to experience pain, waves of nausea,
some difficulty in breathing, and problems with mobility and coherence.
The gamma radiation alarms activated immediately. The blue flash that
they had seen was a result of the Cherenkov radiation that is emitted
when nuclear fission takes place and ionizes air. The addition of the
seventh bucket had caused a self-sustaining chain reaction. The
mixture, in other words, had gone critical. Mixing in the precipitation
tank caused the fissile uranium species to disperse so that the
reaction fizzled out. However, the critical mass later reassembled,
initiating another chain reaction that released more neutrons and gamma
radiation. This cycle was repeated several times over many hours.

None of the three technicians realized what had happened. Mr. Ouchi
had been draped over the top of the tank and was experiencing the
greatest difficulty. The other two workers helped him out of the
building in response to the gamma radiation alarms. A worker in an
adjoining building noticed the injured and confused technicians and
called for medical assistance. An ambulance arrived quickly and removed
the affected workers....

The exact critical mass for the 18.8% uranium mixture in the JCO
precipitation tank is not known.... [T]he critical mass is greatly
reduced when the fuel is in
solution because water acts as a "moderator." Light atoms such as
hydrogen slow the neutrons released by decaying 235U nuclei
between fissions, making it more likely that they will be absorbed and
trigger another nucleus to decay. The critical mass was further reduced
at Tokaimura because a water jacket surrounding the precipitation tank
reflected neutrons back into the tank.... Judging from the levels of
gamma and neutron radiation
measured near the plant perimeter, the criticality excursion seems to
have lasted about 20 hours. After that time, the radiation levels
dropped below detection limits.

The greatest source of radiation exposure in a criticality accident
is the flux of neutrons and gamma rays that emanates directly from the
fissioning nuclei and rapidly decaying fission products.... According
to STA, Hisashi Ouchi was exposed to 17 sieverts of
radiation ... Masato Shinohara was exposed to 10 sieverts of radiation
[and]
Yutaka Yokokawa received a dose of 3 sieverts.... One sievert (Sv),
which equals 100 rems, is a measure of the
biological response to the absorbed radiation. Normally people
receive an average of 0.003 Sv
annually from natural causes. Half of all individuals exposed
instantaneously to 4 Sv die within 30 days. Doses of 10 and 17 Sv are
above the levels normally considered fatal.

Yutaka Yokokawa did not require transfusions and was discharged from
the hospital on December 20, 1999. Hisashi Ouchi, 35, died of multiple
organ failure on December 21,
1999. He suffered serious burns to most of his body, severe damage to
his internal organs, and had a near-zero white blood cell count. Masato
Shinohara, 40, died on April 27, 2000, of multiple organ
failure despite a seven-month fight by doctors after his exposure to
the large dose of radiation.

At least 439 people, including plant workers, firemen, and others
who responded to the accident, and 207 local residents were exposed to
elevated levels of radiation. In October 2000 the total number of
people who received some radiation exposure from the accident was
revised upward to 667. At one of the closest
monitoring sites, STA reported dose rates of 4.5 mSv/hr for neutrons
and 0.50 mSv/hr for gamma rays about 11 hours after the onset of
criticality. That gamma dose rate was about 1000 times higher than the
normal background level."